PredictionIO is an open source prediction engine that allows developers to implement predictive features to their sites/apps.
Such predictive features can be the recommendations of videos, articles, deals, photos or documents, restaurants, etc. that are specific to a given user (which we see in many sites).
The application is designed to work with big data (supports Hadoop, Mahout, Cascading and Scalding) and has SDKs for major programming/scripting languages (Ruby, Python, Java and -soon- PHP).
It has a REST API for integrating the prediction features into apps and an admin interface helps setting up everything.
PredictionIO can be installed on any platform (runs on JVM), it is in the beta phase but still very valuable considering it is hard to find an alternative in that field.